Tuatara are often called living fossils—the ancestors of these New Zealand creatures roamed the Earth 200 million ago and survived the extinction event that took down the dinosaurs. But according to a study released today in the Proceedings of the Royal Society, the lizard-like animal’s long run might come to a sudden end if the planet warms as rapidly as some fear.

The problem is that tuatara, like a lot of reptiles, show what’s called temperature dependent sex determination, meaning that the sex of a baby animal depends on the temperature during its development. For the tuatara, scientists say, the critical temperature is close to 71 degrees Fahrenheit. If the mercury reads higher than that during a baby tuatara’s development, it is much more likely to be born a male. So, the researchers say, a warmer world could throw off the male-female balance.

The scientists, led by Nicola Mitchell of the University of Western Australia, chose New Zealand’s North Brother Island as a testing ground and modeled how severe global warming would affect 52 known tuatara nesting sites there. If the Earth keeps warming, they found, tuatara nesting sites will heat up, too, to the point that the number of females will dwindle, decreasing the species’ chance for successful reproduction.